Digital light deflector using electro-optic grating

ABSTRACT

A digital light deflector or modulator is disclosed in which a polarized sheet of light derived from a laser is directed through a slab of an electro-optic crystal such as lithium niobate, LiNbO3. The crystal slab is thin and has a major surface provided with discrete, regularly spaced electrodes arranged in a column extending transverse to the direction of light passing through the crystal. The opposite major surface of the crystal is provided with a matching, registered column of electrodes. When an electric potential is applied across the two columns of electrodes, an electric field grating is established in the crystal which causes a diffraction of the sheet of light in a direction lying in the plane of the sheet of light. The output sheet of light from the crystal may be translated to a beam of light by lenses.

United States Patent Jacob Meyer Hammer Trenton, N.J. 56,496 July 20,1970 Dec. 7, i971 RCA Corporation [72] Inventor [21] App], No. [22]Filed [45] Patented [73] Assignee [54] DIGITAL LIGHT DEFLECTORUSINGELECTRO- OPTlC GRATING 10 Claims, 4 Drawing Figs.

[52] U.S. Cl 350/160 [51] Int. Cl 602! 1/26 [50] Field 01 Search350/160,

[56] References Cited UNITED STATES PATENTS 3,329,474 7/1967 Harris etal.

Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGrawAttorney-H. Christotfersen ABSTRACT: A digital light deflector ormodulator is disclosed in which a polarized sheet of light derived froma laser is directed through a slab of an electro-optic crystal such aslithium niobate, LiNbO The crystal slab is thin and has a major surfaceprovided with discrete, regularly spaced electrodes arranged in a columnextending transverse to the direction of light passing through thecrystal. The opposite major surface of the crystal is provided with amatching, registered column of electrodes. When an electric potential isapplied across the two columns of electrodes, an electric field gratingis established in the crystal which causes a diffraction of the sheet oflight in a direction lying in the plane of the sheet of light. Theoutput sheet of light from the crystal may be translated to a beam oflight by lenses.

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A 7' TOR/V5 Y PATENTEDUEB H97! SHEET 2 OF 2 INVENTOR. v, Jacob )1.Hammer BY ,@J K 0%- A TTORNEY DIGITAL LIGHT DEFLECTOR USINGELECTRO-OFTIC GRATING BACKGROUND OF THE INVENTION There are many.proposed systems, particularly in the computer and computer memoryfields, which require an electrically operated light deflector capableofdeflecting a laser beam in a digital manner to any one of a plurality ofdiscrete output directions or positions. Known digital light deflectorsare less than completely satisfactory for commercial use because oftheircomplexity, cost, power consumption or slow speed of operation.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF TI-IE'DRAWING FIG. I is aplan view of a digital light deflector constructed according to theteachings of the invention;

FIG. 2 is an elevation of the light deflector shown in FIG. 1;

FIG. 3 is a fragmentary detailed view of a portion of FIG. I; and

FIG. 4 is a perspective view of a deflection system, including thedeflector of FIG. 1, to provide deflection in both X and Y directions.

DESCRIPTION OF THE PREFERREDEMBODIMENT Reference is now made to FIGS. l,2 and 3 where there is shown a laser beam 10, which is directed to anoptical system 12 for translating the laser into a sheet of light havinga large dimension D and a relatively very small dimension E in the otherdirection. The optical system 12 is illustrated as including cylindricallenses l3 and 14, but other optical systems employing spherical lensesor thin film optical waveguide couplers may be used. The optical system12 includes a polarizer 15 so the sheet of light leaving the lens 14consists of parallel or collimated rays of light which are polarized inthe direction Y shown in FIG. 2. The polarized sheet of light havingdimensions D and E is directed to the edge of a slab I6 of electro-opticcrystal material. The crystal material may be lithium niobate, LiNb0,,for example. Another suitable crystal material is strontium bariumniobate, Sr .,,Ba, -,,Nb,0 The slab I6 is lithium niobate electro-opticcrystal is oriented so that an electric field impressed across thecrystal in the direction of the Y axis of the crystal (in the thicknessdirection T) will act to change the index of refraction of light passingthrough the crystal in the X direction (the direction of the widthdimension W), if the incident light is polarized and has its E fieldparallel with the impressed electric field (perpendicular to the majorsurfaces of the crystal slab).

An electric field or fields are impressed on the crystal 16 by means ofa plurality of electrodes positioned on at least one major surface ofthe crystal slab. A first column of electrodes 18 is positioned on thetop surface of the crystal and arranged as a column of equallydimensioned and equally spaced electrodes. A matching column ofregistered electrodes 18' are positioned on the bottom surface of thecrystal slab. All of the electrodes 18 are connected together by a bus22, and all of the matching electrodes 18' on the bottom of the crystalare connected by a bus 22'. The bus 22 is connected through a switch 26to one terminal of a source 28 of electric potential and bus 22 isconnected through a switch 26' to the other terminal of source 28. Thesource 28 may be one providing a potential of about 1,500 volts. Thedescribed elements permit the connection of an electric potential acrossthe electrodes 18 on the top of the crystal and the electrodes 20 on thebottom of the crystal so that an electric field difi'raction grating isestablished in the crystal slab.

Thevdimensions and deflection characteristics of the elecitricfielddeflection grating within the crystal are determined bythe dimensionsand spacings of the electrodes 18 and 18', which are constructed usingwell-known photographic techniques and etching procedures. Theelectrodes 18 and I8 are shown in FIG. 3 to have a dimension d, in thedirection of the column, a spacing also equal to d,, anda dimension a inthe other direction, which is about half of the dimension d The angleof. deflection imparted to a sheet of light passing through the crystal,represented by the angle 0 in FIG. 1, is related to the dimension andspacing of the electrodes I8 by the formula:

firm/2dr) where A is the wavelength of the light. The electrodes of acolumn need not beequally dimensioned and spaced but may be dimensionedand spaced according to some other periodic array such as one in whichthe a dimension varies in the z direction according to the formula:

sin wZ/d,

In this case the index of refraction in the crystal will have asinusoidal variation in the 2 direction and will tend to give a largepercentage of first order grating spectra.

The column of electrodes is shown as being at right angles with thedirection of the incident beam. The column of electrodes may be at othertransverse angles with the beam, and

may, for example, be at the: Bragg angle. In this case if the dimension0 of the electrodes is also greatly extended, higher order diffractioncomponents are absent and substantially all of the light of the incidentbeam can be deflected.

Theelectric field grating produced in thelithium niobate crystalby theelectrodes actsas a phase grating on an incident light beam polarized inthe direction of the Y axis shown in FIG. 2. According to anothermode ofoperation, the electric field grating acts as a polarization gratingwhen the incident light is polarized in a direction approximately 45from the direction of the Y axis. When different crystal materials areused, the orientations of the crystal axes in relation to thepolarization of the incident light and the direction of the electricfield should be chosen following well-known criteria to provide a phasegrating or a polarization grating, as may be desired.

It is thus far apparent that, in the absence of the application of anelectric potential to the columns of electrodes 18 and 18', the incidentsheet D of light passes directly through the crystal to output positionF, but that when an electric potential is applied to the electrodes, theoutput sheet of light is deflected an amount represented by the angle 0to output position G.

The crystal slab I6 is also provided with a second column of registeredelectrodes 32 and 32' which have a larger dimension d, and an equallylarger spacing also equal to d,. The more coarsely dimensionedelectrodes 32 and 32' of the second column produce a coarser electricfield diffraction grating in the crystal than the electrodes 18, 18' ofthe first column, and therefore the electrodes of the second columnproduce a smaller amount of deflection when energized than theelectrodes of the first column. The slab I6 is also provided withadditional third, fourth and fifth columns of electrodes 34, 34, and 36,36' and 38, 38', having progressively larger dimensions to provideprogressively smaller amounts of deflection of the light passing throughthe crystal. The crystal slab 16 is thus shown to be provided with fivesets or columns of electrodes any one of which may be energized from thesource 28 through switches to provide respective different amounts ofdeflection of a sheet of light passing through the crystal.

An additional number of difierent discrete angles of deflection can beachieved by energizing more than one of the columns of electrodes at atime. The energization of every different combination of columns ofelectrodes results in a corresponding different output angle ofdeflection from the crystal. The number of different directions in whichthe output light can be deflected is equal to 2" where n is the numberof columns of electrodes on the crystal. For example, when there arefive columns of electrodes as shown in the drawing, and all combinationsof columns can be energized, the light can be deflected to 32 differentdirections. Therefore, a large number of difi'erent deflection anglescan be obtained with relatively few columns of electrodes.

According to a simpler, but less efficient construction, the bottomelectrodes 18', 32', 34', 36' and 38 are replaced by a continuousconductive electrode which extends over the bottom surface of thecrystal and is permanently connected to ground.

The deflection system shown in FIGS. 1 and 2 produces an output sheet oflight having one of many desired deflection angles in a direction lyingin the plane of the sheet of light. Since it is frequently desired toproduce a deflected output light in the form of a beam of light, thesystem shown in FIGS. 1 and 2 can normally be followed by conventionaloptics (not shown) which is constructed to translate the deflected sheetof light to a deflected beam of light. For this purpose, a beamcompressor may be employed which is the inverted equivalent of the beamexpander l3, l4 acting on the incident light beam 10.

Reference is now made to F IG. 4 for a description of an opticaldeflector according to the invention which is capable of deflecting abeam of light by any one of many discrete amounts in one direction, andthen deflecting the resulting light by any one of many discrete amountsin an orthogonal direction. The optical system 12 and the crystal slab16 are the same as the corresponding elements shown in FIGS. 1 and 2.The light output from the crystal 16 is passed through an optical system50 which translates the vertically oriented sheet of light from thecrystal 16 to a correspondingly deflected horizontal sheet of light forapplication to a stack 52 of deflectors each of which is like thedeflector 16. There are as many deflectors in the stack 52 as there areoutput deflection angles from the stack 16. That is, five differentcolumns of electrodes shown on the deflector 16 provide five differentangles of deflection therefrom. The stack 52 of deflectors includes fiveseparate deflectors each like deflector 16.

When the first column of electrodes is employed to produce a givendeflection from deflector 16, the deflected light is made to go throughthe first deflector in the stack 52. Similarly, when the second columnof electrodes and deflector 16 is energized to produce a different angleof deflection, the deflected light goes through the second deflector inthe stack 52. In like manner, the third, fourth and fifth columns ofelectrodes in deflector 16 result in deflected light going through thethird, fourth and fifth deflectors in the stack 52. A column ofelectrodes in any particular one of five deflectors in stack 52 can beenergized to produce an additional deflection of the light in adirection orthogonal to the direction of deflection produced bydeflector 16. Since it is known when a particular one of the columns ofelectrodes in deflector 16 will be energized, solely a desired one ofthe columns of electrodes in solely one of the deflectors in the stack52 needs to be energized.

Therefore, the power consumption required to achieve the deflection intwo directions is only twice the amount needed for deflection in asingle direction.

What is claimed is: 1. An optical deflector or modulator, comprising aslab of electro-optical material having major opposite surfaces,

a periodic array of spaced electrodes arranged in a column on a firstmajor surface of said slab. electrode means on the other opposite majorsurface of said slab, means to direct an incident sheet of light havinga given polarization through said slab between said column of spacedelectrodes, and means to establish an electric potential differencebetween the electrodes on the first surface of said slab and theelectrodes on the opposite surface of said slab, whereby an electricfield diffraction grating is established in said slab which causes thesheet of light emerging from said slab to be deflected a given amount.

2. A deflector as defined in claim 1 wherein said incident sheet oflight is directed through said slab in a direction transverse to saidcolumn of electrodes.

3. A deflector as defined in claim 1 wherein said electrooptic crystalmaterial is lithium niobate, and said incident sheet of light ispolarized in the direction perpendicular to said major surfaces of thecrystal slab.

4. A deflector as defined in claim 1 wherein the electrodes in saidarray of electrodes are equally dimensioned and equally spaced.

5. A deflector as defined in claim 1 wherein said electrode means on theopposite major surface of the slab consists of matched electrodeslocated in registry with the electrodes on said first major surface.

6. A deflector as defined in claim 1, and, a plurality of additionalcolumns of electrodes each having progressively different dimensions andspacings, whereby additional different amounts of deflection can beimparted to the sheet of light.

7. A deflector as defined in claim 6 wherein said electrode means on theopposite major surface of the slab consists of matched electrodeslocated in registry with the electrodes on said first major surface.

8. A deflector as defined in claim 6, wherein said means to establish anelectric field potential difference between the electrodes on the twosurfaces of the slab includes switch means to energize any combinationof said column of electrodes, whereby to produce as many differentangles of deflection as there are combinations of column.

9. A deflector as defined in claim 8 wherein the electrodes in eachgiven column are equally dimensioned and equally spaced.

10. A deflector as defined in claim 7, and, in addition, a stack ofdeflectors each as defined in claim 7, the deflectors of said stackbeing located to receive output light from said deflector and beingarranged with major surfaces at right angles to the major surfaces ofsaid deflector, and optical means to rotate the sheet of light from saiddeflector so that it enters the input edges of deflectors of said stack,whereby to provide deflection of light in two dimensions.

2. A deflector as defined in claim 1 wherein said incident sheet oflight is directed through said slab in a direction transverse to saidcolumn of electrodes.
 3. A deflector as defined in claim 1 wherein saidelectro-optic crystal material is lithium niobate, and said incidentsheet of light is polarized in the direction perpendicular to said majorsurfaces of the crystal slab.
 4. A deflector as defined in claim 1wherein the electrodes in said array of electrodes are equallydimensioned and equally spaced.
 5. A deflector as defined in claim 1wherein said electrode means on the opposite major surface of the slabconsists of matched electrodes located in registry with the electrodeson said first major surface.
 6. A deflector as defined in claim 1, and,a plurality of additional columns of electrodes each havingprogressively different dimensions and spacings, whereby additionaldifferent amounts of deflection can be imparted to the sheet of light.7. A deflector as defined in claim 6 wherein said electrode means on theopposite major surface of the slab consists of matched electrodeslocated in registry with the electrodes on said first major surface. 8.A deflector as defined in claim 6, wherein said means to establish anelectric field potential difference between the electrodes on the twosurfaces of the slab includes switch means to energize any combinationof said columns of electrodes, whereby to produce as many differentangles of deflection as there are combinations of columns.
 9. Adeflector as defined in claim 8 wherein the electrodes in each givencolumn are equally dimensioned and equally spaced.
 10. A deflector asdefined in claim 7, and, in addition, a stack of deflectors each asdefined in claim 7, the deflectors of said stack being located toreceive output light from said deflector and being arranged with majorsurfaces at right angles to the major surfaces of said deflector, andoptical means to rotate the sheet of light from said deflector 90* sothat it enters the input edges of deflectors of said stack, whereby toprovide deflection of liGht in two dimensions.